MOSFET TESTING

You can do the most basic test with a 6 or 9V battery, a 6k8 resistor & LED.

Put these items in series to make a simple continuity tester - obviously the battery + & minus termials will point to ends of the series combination, call those ends + & minus respectively.

1) place the minus on the source lead & + on the drain - the LED might partially light due to static on the gate. 2) keep the minus lead on source and touch the + lead on the gate to charge the gate capacitance - the move + back to drain, the LED should light full bright to show fully conducting channel - if it slowly fades out the gate isolation is probably damaged. 3) put the + lead on the source and touch the minus lead to the gate to charge the gate capacitance with a negative voltage - that should switch the MOSFET completely off, so when you repeat the minus to source/+ to drain test; any glow in the LED means leaky.

With P-channel MOSFET the above sequence is repeated with the polarity reversed.

With todays DMM's that test, in my opinion is bogus. The best you can maybe expect is to drain the gate down a bit from where it is and there for it just turns off the mosfet. Unless of course, you have a unit with a very low Vg(th), but that does not appear to be in your case.

You see the problem is, circuits operate at low voltage and it is not good to be putting voltages much over 1.5 in the circuit.

The mosget you have there requires 3 volts at the gate. I don't think your DMM is generating that much, other wise, it could be damaging when using the DIODE mode for in circuit testing..

At one times they did 2 volts with a diode test. I am not sure what they do now. BUt you at least have to get a good margin above a diode drop.

What I do when I am not sure about the Vg(th) value, or if the unit even works properly. Is use a Resistor from a power supply that can do at least 12 volts for example.. Like below..

Variable voltage

+-----------+------------+ + + | /+\ | ( ) 12V | \-/ + | .-. | | | 1k === | | GND '-' V Meter | _ + / \ | o----------------o------------o-(_/_)-=== | | D \_/ GND | | | + | ||-+ | G ||

Jamie, you recommended what I was going to, set up a circuit for the Mosfet. But I would have put a *voltage divider on the gate.

10k from 12v to gate and another 10k from gate to ground, then put a switch from gate to ground. With this she can watch the Mosfet switch on/off with changes in the position of the switch. Maybe even use a 100 ohm or 10 ohm from 12 v to Drain to get a little current flowing. Just my two cents. Mikek *this assumes removing the gate to 1k/drain connection in your circuit.

Your link to the data sheet didn't work for me, but this does:

I use that trick sometimes, and it is quite useful. I use the diode = check=20 which may put out as much as 3 volts or more (which is useful for LEDs = as=20 well), and then check the D-S resistance. Then short G-S and read open = in=20 one direction and the body diode in the other. It is good to see if the=20 input gate resistance is high enough to hold the charge while you take = the=20 reading.

As noted, your MOSFET has a gate threshold of 3-5 volts, so you will = need=20 something like a 9V battery to get enough voltage. You COULD charge a=20 capacitor with the multimeter voltage and then put it in series with the =

multimeter to the gate to get 4-6V. Just in case you didn't have a spare =

battery but had a capacitor handy.

Paul=20